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When I'm trying to figure out how much to water things in my garden I'll often see things like, "...they will need 1-2 inches of water each week."

From what I understand, this is the same as if there had been 1-2 inches of rain. My problem is that everything for drip irrigation systems seem to measure in gallons or liters per hour.

How can I go about converting inches to gallons?

5 Answers 5

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I just went through the same thing myself when setting up my drip irrigation system. It's actually pretty easy.

The formula is as follows:

Pr = 96.25 x Total GPM of all emitters (gal/min)
     -------------------------------------------
                 Total area (ft²)

Where Pr is the "precipitation" rate in inches/hour.

To make it more clear, I'll take an example from my own garden. I have 6 slicing tomato plants planted in roughly an 18' x 3' area. I have 2 sections of drip tubing that stretch on either side of the plants which provide a total of 29 0.9 gal/hour (GPH) emitters within that area. I also have an additional single 1 GPH emitter for each plant.

So, to find the Total GPM of all emitters (gal/min) I do the following:

29 x 0.9 gal/hr = 26.1 gal/hr

6 x 1.0 gal/hr = 6 gal/hr

26.1 gal/hr + 6 gal/hr = 32.1 gal/hr

To get gal/min, divide by 60:

32.1 gal/hr / 60 = 0.535 gal/min

So, plug that into the equation and you get

Pr = 96.25 x 0.535 gal/min
     ---------------------
            18 x 3

So, Pr = 51.5 / 54 or .954 inches/hour

Let's say I want to give them 2 inches of water per week. At 0.954 in/hr, I'd need to water a total of 2.097 hours, or 126 minutes per week.

I compared this calculated method to an actual physical test (held tub underneath 2 of the drip tape emitters for 30 minutes), and it's pretty much dead on.

I highly recommend setting this up as a spreadsheet with columns for width and length, and then columns for the quantities of each speed emitter you have.

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  • Possible simplification when using evenly spaced drip tape or tube down rows with identical emitters everywhere: 96.25 * GPM per emitter / (feet between rows * feet between emitters). Nice thing about this simplification: you don't need to measure the garden. For example, if you use drip tape with 0.5 GPH emitters every 1 foot on 6 foot rows, 96.25 * (0.5/60) / (1*8) = 0.1337 inches per hour, and this holds for any garden size.
    – Phil Frost
    Jun 16, 2021 at 17:40
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One US gallon is 231 cubic inches.

One square yard is 1296 square inches.

So "1 inch of rain" is 1296/231 or about 5 and a half US gallons or 21 liters of water per square yard.

The flow rate through your drip system obviously depends on the area it is watering and how many hours it is on each day, but that gives you a starting point.

Note: 1 imperial (UK) gallon is 277 cubic inches which gives 4 and three quarter UK gallons per square yard - but commercial UK irrigation systems will be specified in metric units so that is fairly irrelevant.

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An Imperial Gallon is about 4.5 litres. An inch is 0.025 metres. So an inch of water per week is equivalent to 0.025 cubic metres, or 25 litres per hour per square metre for one hour, or 5.6 gallons per hour per square metre for one hour. It's early morning here, so I hope I've got that right.

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  • You forgot to say how much ground the "2.2 gallons per hour" will water. (Note to self: don't answer questions before being fully awake...)
    – alephzero
    Jul 22, 2019 at 9:12
  • @alephzero - I thought I might screw it up. I've now added "per square metre". I'll have you know that not only did I get the figures wrong, I also assumed the OP was Canadian (hence my reference to Imperial Gallons) when I saw he comes from Sacramento, CA. Whoops.
    – Peter4075
    Jul 22, 2019 at 12:23
  • 1
    Just to be clear, "25 liters per hour for one hour" is the right number so long as the system is turned off for the other 167 hours in the week. But "an inch of rain in one hour" is a severe tropical storm, not the usual way to water plants!
    – alephzero
    Jul 22, 2019 at 20:09
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One inch times one square foot is about 0.623 gallons. So, if you have S square feet to water with N inches (say per week), that is

0.623 * S * N

Or:

(0.623 gallons / square foot / inch) * (square feet) * (inches) = gallons

As an example, if you're watering three tomato plants that need 1.5 inches per week each, and you estimate the area you're watering around each plant to be about a square foot (1' x 1'), that's 0.623 * (1+1+1) * 1.5 = 2.804 gallons.

Note: I have no practical experience, I just wanted to write down the formula I was hoping to find.

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It's byproduct of volume. A gallon of water is 7.48 cubic feet or 12,925 Cubic inches. Surface area if collection surface in square inches divided by 144 (1 square foot)

Inch divided by inches in a Total foot equals X/12 multiplied by inches of rain. That's your cubic footage...

Multiply by 7.48 equals gallons

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